European Patent No. 0 469 207 describes a circuit arrangement that is equipped with load resistors which are assigned to current conductors and provide a separate measuring potential input for each secondary phase winding. Diodes that establish a connection to the common voltage-stabilized Zener diode and supply the operating voltage via a charging capacitor are connected downstream from the secondary windings. Generation of the operating voltage is decoupled from generation of the measuring signals by diodes.
As described in World Patent No. 90/12342, decoupling is also achieved by bypass circuits, in which the power supply system is decoupled from the current peak values to be measured.
As described in European Patent Application No. 0 687 050 the operating voltage is generated and the current detected by analog-controlled switching elements which are also supplied individually for each current conductor. In this regard, pure a.c. measurements using evaluation circuits are also described in which a separate load resistor is provided for each current conductor. Because the load resistors also have resistance values that are subject to specific tolerances, the measurement results obtained during current detection are imprecise.
Furthermore, a power supply unit for overcurrent tripping devices which control the necessary operating voltage in conjunction with a microprocessor is described in German Patent Application No. 32 23 687. However, the measuring circuit used in this case to detect the current is not decoupled from the circuit for generating the operating voltage, which means that mutual disturbances occur, resulting in imprecise measurement results.